One common type of electrical power converter that produces a regulated output voltage is a switch mode power supply or a switched supply. Conventional switch mode power supplies commonly include a power transformer and one or more power switches for alternately coupling a DC voltage across a primary winding of the power transformer, thereby generating a series of voltage pulses across one or more secondary windings of the power transformer. These pulses are then rectified and filtered to provide one or more output DC voltages.
The size, cost, and electrical performance of conventional transformers are key limitations of switch mode power supply designs. An ideal transformer for switch mode power supplies would be compact (low profile); would efficiently transfer energy from the primary windings to the secondary windings; would have minimal leakage inductance; and would be manufacturable.
Conventional transformers are generally manufactured by winding a primary coil of insulated wire on a bobbin, while a secondary coil, also of insulated wire, is wound on another bobbin. The transformer core typically consists of two segments that can be attached together. The two attached segments form a hollow section, or winding window, in which the transformer coils are situated. The transformer is typically assembled by arranging the two bobbins concentrically in the winding window of the segments of a transformer core and then attaching the segments of the transformer together around the bobbins.
It is desirable that transformers for switch mode power supplies be of minimal size, both in terms of cross-sectional area and in terms of winding height. A fundamental limitation on transformer performance results from Faraday's law. According to Faraday's law, the induced voltage across each secondary winding turn of a transformer is proportional to the time rate of change of the total magnetic flux crossing the secondary winding turn. The transformer size can be reduced by decreasing the number of winding turns or reducing the cross-sectional area of the transformer. However, if the number of winding turns and the area of each winding turn is decreased, then the magnetic flux density swing and the frequency of operation must increase in order to maintain a constant induced voltage across the secondary winding. Transformer core losses increase rapidly with magnetic flux density. Eddy current losses increase with the square of the magnetic flux density. Hysteresis losses also obey an exponential relationship, typically increasing as the magnetic flux density raised by an exponent in the range of 1.8 to 2.5, depending upon the core material. Consequently, the peak magnetic flux density in the transformer core is typically limited to less than 1 Tesla in conventional transformer designs to limit the heating and loss of efficiency caused by eddy current and hysteresis losses.
Increasing the switching rate, or frequency, is one common technique used to decrease the size of transformers used for switch mode power supplies. However, the efficiency of transformers degrades at high frequency (e.g., frequencies on the order of 1 MHZ) because of increased resistive losses in the primary and secondary windings. Classical electromagnetic theory teaches that at high frequency the current distribution in a wire decreases exponentially with a characteristic length, or skin depth, from the surface. The skin depth varies inversely as the square root of the frequency and the conductivity of a metal. For example, at a frequency of 1 MHZ, the skin depth decreases to 66 .mu.m, such that only a small annulus of a wire conducts. The effective cross-sectional area for current flow thus decreases dramatically at high frequency, leading to a corresponding increase in resistance of the primary and secondary windings. Moreover, the problem of increased resistive losses in the secondary windings at high frequency is exacerbated when magnetic field strengths are high, because proximity effects further limit the effective cross-seclional area of the secondary windings.
Another limitation to high frequency operation of a low-profile transformer is leakage inductance. The leakage inductance occurs because not all of the of the magnetic flux generated by the primary winding is coupled by the core to the secondary winding. Some of the magnetic flux generated by the primary winding does not intersect the secondary winding but instead passes through the air space around the sides of the primary and secondary windings. In the equivalent circuit model of a transformer this leakage flux is modeled as a corresponding parasitic leakage inductance that must also be driven by the primary current but which does not couple power to the secondary winding. The transformer leakage inductance thus has the effect of impeding the flow of power from the primary winding to the secondary winding. As the switching frequency is increased, the deleterious effect of the leakage inductance increases. The leakage inductance can be reduced by spacing the primary and secondary windings as close to each other as possible, which has the effect of increasing the relative fraction of magnetic flux coupled to the secondary winding while reducing the relative fraction of leakage flux passing through the air space. Alternating the primary and secondary winding turns, what is commonly known in the art as interleaving, can also aid in bringing the primary and secondary windings close to each other, resulting in reduced leakage inductance.
Another limitation to high frequency operation of low-profile transformer is eddy current losses in the windings. The magnetic field in the air space between the windings is created by the currents flowing in both the primary and secondary windings. At high frequencies, the magnetic field caused by these current flows creates eddy currents in the windings, leading to undesirable losses. However, if the primary and secondary winding are interleaved, then there can be a substantial canceling of the magnetic field that creates these eddy current losses, leading to improved performance.
International safety standards impose additional limitations on transformer design, further exacerbating the above-described problem of miniaturizing a transformer while maintaining strong coupling between primary and secondary windings. International safety standards exist for "creepage"; "clearance"; and minimum insulation thicknesses. "Creepage" is defined as the shortest distance between two conductive parts (or from a conductive part to ground) as measured along the surface of the insulation. "Clearance" is defined as the shortest distance between two conductive parts (or between a conducting part and ground) as measured through air. For transformers used in typical switch mode power supplies, the minimum creepage distances established by international safety standards is at least 4 mm. International safety standards also require that the primary and secondary windings be separated by either 3 layers of insulation or a single layer greater than 0.4 mm thick. The protective insulation layers should also not be mechanically stressed. For a given winding topology, the insulation and creepage requirements imposed by international safety standards increases the minimum separation between primary turns; reduces the maximum number of primary turns for a given winding height; and increases the separation between primary and secondary windings. Consequently, international safety standards exacerbate the problem of achieving a very low-profile design with strong coupling between the primary and secondary windings.
Several approaches in the prior art exist for solving some of the above identified problems, although none is a completely satisfactory solution to achieve a low profile transformer consistent with switch-mode applications. For example, the prior art describes changes in winding topology to minimize eddy currents in the windings in conventional transformers with wound-wire bobbins. Changes in transformer topology can beneficially alter the magnetic field distribution, resulting in a more uniform magnetic field strength distribution. In particular, by interleaving the primary and secondary windings, the peak magnetic field strength is reduced in the air space between windings. However, an extremely low profile interleaved transformer design for switch mode power supply applications is not practical with conventional winding approaches because safety insulation requirements impose large interwinding distances, leading to poor coupling of primary and secondary windings. Even variations on conventional winding schemes suffer from the same problem. For example, the approach of U.S. Pat. No. 5,473,302 (entitled "Narrow Profile Transformer Having Interleaved Windings And Cooling Passage") describes a narrow profile transformer in which the primary and secondary windings consist of interleaved spirals comprised of insulated primary and secondary winding wires. However, such an approach would result in high resistance losses for high frequency operation because conventional wires are used for the windings. Additionally, this design is unsuitable for switch mode power supply applications. The coupling between primary and secondary windings will be poor because of the large physical separation between primary and secondary winding wires imposed by international safety requirements.
The prior art also describes low profile transformers in which the secondary winding is replaced with at least one stamped conductive foil sheet. Such an approach is described in U.S. Pat. No. 5,175,525 (entitled "Low Profile Transformer"). The primary winding consists of an encapsulated wire winding. The secondary foil windings, also encapsulated, are arranged coaxially with the primary winding. This approach has the advantage of reducing the high frequency resistance of the secondary winding since the current can flow in a broad sheet in the secondary winding. However, the coupling between the primary and secondary windings, while high because of the coaxial arrangement, is degraded by the large separation between windings necessitated by the individual encapsulation of each winding. Moreover, the high frequency resistance of the primary winding will be larger than ideal for applications where a large diameter primary winding wire is typically used. For example, a primary winding designed for a 30 V input voltage might comprise 3 turns of AWG22 magnet copper wire that has a wire diameter of 0.64 mm, which is much greater than the skin depth of copper at a switch-mode frequency of 500 kHz. Additionally, since the design is not interleaved, the eddy current and hysteresis losses will be high.
The prior art also describes low profile transformer designs in which all of the wire windings are replaced by completely planar windings. For example, in the approach of U.S. Pat. No. 5,179,365 (entitled "Multiple Turn Low Profile Magnetic Component Using Sheet Windings") conventional wire windings are replaced with copper sheets each stamped into the shape of a circular annulus, with each annulus replacing one turn of wire. This has the advantage that the high-frequency resistance of the windings is reduced, since the current in each winding flows in a broad cross-sectional area across the annulus instead of only the short circumferential skin depth of a conventional wire. Also, in principle, it is possible to interleave primary and secondary winding sheets with this approach. However, while many annular sheets of copper can be combined to create a "sandwich" of windings, there are many complications. First, each winding sheet much be connected to other sheets with appropriate pins and connectors for mechanical support and to create the required electrical connections, e.g., an n-turn primary must connect n-sheet windings. Second, mechanical considerations limit how thin a sheet of copper can be with this technique. The copper thickness must be thick enough to provide mechanical rigidity, which will tend to be much thicker than the optimum conductor thickness. Third, if such a design was used in a switch mode power supply, additional layers of insulation would have to be incorporated in order to meet international standards for creepage, clearance, and insulation. The resulting transformer would be complicated to manufacture and have a larger than ideal separation between the windings, resulting in a poor coupling of the primary and secondary windings.
In another low profile transformer approach, planar windings are created on printed circuit boards. In the approach described in U.S. Pat. No. 5,010,314 (entitled "Low-Profile Planar Transformer For Use In Off-Line Switching Power Supplies"), primary winding turns are patterned on two or more printed circuit boards, and secondary winding turns on one or more printed circuit boards. A compact transformer can be created by stacking several such printed circuit boards together in a sandwich configuration, with each winding separated by insulating layers composed of the printed circuit board itself and additional insulation (if required to meet safety standards) applied to the surface of each patterned winding. However, this approach suffers from numerous drawbacks. First, the thickness of conducting metals that can be patterned or plated has practical limitations such that it is difficult to pattern conducting layers comparable to the skin depth (at common switch mode power supply frequencies) in order to obtain minimum resistance losses in each planar winding. For example, as previously discussed, at a frequency of 1 MHZ the skin depth of copper is 66 .mu.m. In order for planar winding turns to have minimum resistance (and since two sides of the surface conduct), a film thickness in excess of 132 .mu.m is required, a thickness that is difficult to conveniently pattern with existing techniques. Second, it is necessary to electrically connect different layers of the sandwich in order to create an electrically continuous primary or secondary "coil" from multiple layers. Via hole connections or additional external connecting rods must be used, increasing the manufacturing problems of this approach. Third, in order to satisfy international standards on creepage, the inner-most winding must be separated at least 4 mm from the central core, resulting in a larger than ideal transformer area. Fourth, the coupling of the primary and secondary windings may be degraded if the thickness of the printed circuit boards, required insulation, and necessary spacers creates a larger than ideal separation between layers.
Another approach to fabricating low profile transformers with planar winding turns consists of folding a patterned sheet upon itself to convert a two-dimensional pattern into a set of coaxial coil-like windings. This approach is described in U.S. Pat. Nos. 4,959,630 (entitled "High-frequency transformer"), 5,084,958 (entitled "Method Of Making Conductive Film Magnetic Components"), and 5,017,902 (entitled "Conductive Film Magnetic Components"). Conducting paths with a repeating (periodic) serpentine shape are patterned on both top and bottom sides of a planar but flexible film. The patterned film is then folded upon itself along each half-period of the serpentine. The accordion-like folding after each half-period creates a series of spatially concentric half coils, with, for example, each full primary winding traversing a 180 degree turn on one segment of the film and completing another 180 degree turn on another, now accordion folded, segment of the flexible film. Because both sides of the flexible film are patterned with serpentine shaped conductors, a series of concentric primary and secondary windings are formed. However, this approach also suffers from several drawbacks. First, there are practical limits on the thickness of the patterned conductor, both in terms of the patterning and the folding process, making it difficult to achieve optimum conductor thickness. Second, there is the cost and difficulty of fabricating such flexible circuits and in enclosing the windings in suitable insulation that meets international safety standards. It is difficult to satisfy international safety requirements because there are creepage paths from the winding turns to the transformer core and between the primary and the secondary winding. Third, there are mechanical problems with this approach, because folding the film back upon itself creates mechanical stresses at the sharp "accordion" edges of the film, which increases the likelihood of insulation breakdown. The problem of mechanical stress at the accordion folds is exacerbated because the folds are located inside of the assembled transformer and thus subject to the mechanical stresses resulting from the transformer assembly process in addition to those stresses associated with the thermal cycling of the transformer. Fourth, additional contacts or solder joints are needed to connect the coils to external contacts. Although each patterned serpentine has two ends, once it is accordion folded, one end of each coil will be folded under another layer with only a narrow cross section of the conductor exposed at the edge of the fold. Consequently, to make electrical contact to each folded-under end of a coil will require soldering or bonding contacts at the edge of the folds, exacerbating the manufacturing and reliability problems.
None of the existing approaches for low profile transformers is a fully satisfactory solution to the problem of designing low profile transformers for switch mode power supplies. All of them have manufacturing problems in addition to design problems that can severely degrade their performance for switch mode power supply applications. No known prior art transformer design possesses all of the desired characteristics for a low-profile design: 1) minimally spaced interleaved primary and secondary windings to achieve a high coupling factor, low eddy currents, and low leakage inductance; 2) wide planar windings of optimum thickness (greater than the skin depth) to minimize high-frequency resistance; and 3) a manufacturable design consistent with international safety standards. Consequently, there is a need for an improved transformer design that is compact (low profile); high efficiency (minimal resistive losses and core losses); is consistent with international safety requirements for creepage, clearance, and insulation; and that can be economically fabricated.